Ion accelerators



.zvi-mcl SR O l/ MTFQQQ XR 391439650 WW Wa Aug. 4, 1964 S. KLEIN E'I'ALION ACCELERATORS Filed Nov. 24, 1959 United States Patent 3,143,680 IONACCELERATORS Siegfried Klein, Paris, and Jacques Pottier, Orsay, France,

assignors to the Commissariat a lEnergie Atomiqne,

Paris, France, an organization of France Filed Nov. 24, 1959, Ser. No.855,176 Claims priority, application France Nov. 25, 1958 14 Claims.(Cl. 313-63) The present invention relates to ion accelerators and moreespecially to linear accelerators of the electrostatic type whichinclude an evacuated tube in which the ions produced by an ion sourceand introduced into the tube at one of the ends thereof are acceleratedby an electrostatic field in the direction of the other end of the tubetoward a target.

In the known linear electrostatic accelerators, the field is essentiallyproduced by a very high direct potential dilference applied between theends of the evacuated tube. However, in order to facilitate insulationand to achieve a constant electrostatic field, the difierence ofpotential is generally divided into several sections by making use ofseveral metallic elements or electrodes provided inside the evacuatedtube and brought to potentials which increase gradually in absolutevalue in the direction of the target, for instance by connecting themwith successive points of a chain of resistors in series forming a kindof potentiometer between the two terminals of the very high potentialsource. Such electrostatic accelerators have the drawback of requiring asource of very high potential and therefore a source of very largedimensions such as the Van De Graatf generator.

On the other hand, there are known linear accelerators in which theacceleration of the ions is obtained by means of high frequencyalternating electromagnetic waves applied to electrodes located insidethe evacuated tube and the potential of which therefore undergoesalternating variations. Such accelerators do not require feed sources atvoltages as high as the electrostatic accelerators above referred to butthey can accelerate only a single kind of ions due to the fact that thegeometrical characteristics of the accelerator and the electricalcharacteristics of the feed source are determined in accordance with themass and the charge of the ions to be accelerated.

The purpose of the present invention is to provide an ion acceleratorwhich, on the one hand, requires only a relatively low voltage source asin the case of the above mentioned electromagnetic wave accelerators sothat it can be made of small dimensions and, on the other hand, permitsof accelerating ditferent kinds of ions as in the case of the abovementioned electrostatic accelerators, owing to the fact that thevelocity of the ions at different points of the accelerator has noinfluence upon the operation thereof.

With this object in view, the apparatus according to the presentinvention, intended to accelerate bunches of ions supplied at areference potential by a high intensity pulsating ion source, includesan evacuated tubular chamber one end of which receives ions from saidion source, a series of electrodes partly permeable to the ions to beaccelerated and disposed behind one another in said chamber, a directhigh voltage source, a series of resistors of high resistance the numberof said resistors being equal to that of said electrodes, conductormeans for connecting one of the terminals of said high voltage sourcewith each of said electrodes through one of said resistors respectivelyand conductor means for connecting the other terminal of said highvoltage source with the ground corresponding to said referencepotential. We thus obtain an ion accelerator, especially for heavy ionseither positive or negative, which has the following properties:

(a) It permits of applying very high accelerations to the ions due tothe fact that each of the acceleratingelectrodes, normally brought to apotential which is high in absolute value, has its potential suddenlyreduced in absolute value just before it is struck by the ions, thispotential becoming practically zero when the ions reach the electrode,which then produces between this electrode and the next one a difierenceof potential equal to the high voltage that is applied, whereby it ispossible to obtain by recurrence a total acceleration potential nearlyequal (account having to be taken of the losses) to the product of saidhigh voltage by the number of electrodes; and

(b) The apparatus is self regulating due to the fact that the time oftransit or flight of the ions from one accelerating electrode to thenext one (which time depends upon the mass and the charge of the ions)has no influence upon the operation of the device, contrary to whattakes place in the case of an acceleration by means of high frequencycurrents.

The ion accelerator according to the invention requires a source capableof supplying intensive ion bunches, for instance an ion source of thetype described in the United States patent application Ser. No. 840,429,filed September 16, 1959, now US. Patent No. 3,015,745 to SiegfriedKlein, issued January 2, 1962, due to the fact that a sufiicientintensity (of at least one ampere) is necessary to achieve within asufiiciently short period of time the voltage drop Which brings thepotential of the electrode struck by the bunch of ions down to groundpotential and also due to the fact that the intensity of the ion beam isgradually reduced by its impact upon successive electrodes. This is whyit is preferred to have an ion source of very high intensity averagingsome tens of ampere.

Preferred embodiments of the present invention will be hereinafterdescribed with reference to the accompany ing drawings, given merely byway of example and in which:

FIG. 1 show in diagrammatic sectional view a linear ion accelerator madeaccording to the present invention and used in connection with an ionsource made accord ing to the above mentioned prior patent application.

FIG. 2 is a front view of one of the accelerating elec trodes shown inthe form of a grid on FIG. 1.

FIG. 3 shows a modified accelerating electrode.

According to the present invention, the accelerator, intended toaccelerate ion bunches supplied by a pulsating ion source S of highintensity includes a tubular chamber 2 evacuated by means of a pump 3and the end 1 of which receives bunches of ions from said ion source(said ions being possibly given an initial acceleration by means of anextracting electrode 4). The apparatus further includes a series ofaccelerating electrodes 5, 6, 7, 8, either in the form of grids g asshown in front view on FIG. 2 or of discs p provided with a central hole0 as shown by FIG. 3, a movable diaphragm 9 (adapted to be replaced bythe target to be bombarded by the high energy ions supplied by theaccelerator) being disposed at the other end of tubular chamber 2. Oneof the terminals 10 of a high direct voltage source 11 (averagingseveral tens of kilovolts) is connected through a conductor 10, and aline 12 with each of the accelerating electrodes 5, 6, 7, 8, through aresistor 13, 14, 15, 16, respectively, of high ohmic value (of the orderof magnitude of l megohm) with the provision of insulating elements 5,,6 7,, 8 for the passage of the conductors through the wall of chamber 2,the other terminal 17 of source 11 being brought by means of conductor17,, to the reference potential R owing to the connection of saidconductor with a grounding part consisting either of the wall of chamber2 when said wall is made of a conducting material, or preferably of aseparate element when the wall of chamber 2 is made of an insulatingmaterial.

Source S may include, as described in the above mentioned prior patentapplication, a non-metallic flexible tube 18 one end of which is fittedon a metallic tube 19 of short length having a throttled part 20 andwelded at 21 to the open tubular end of a bulb 22 made of aluminum andsodium bore-silicate glass such as designated by the trademark Pyrex.This bulb is also open at the other end 23 to form a tube whichpenetrates into a metallic ring 24 provided at the end 1 of theaccelerator chamber 2, a flexible toroidal-shaped packing member 25 (forinstance of rubber) being interposed between bulb 22 and chamber 2.

In order to ensure ionization in bulb 22, there is provided about saidbulb a self-inductance coil 26 (of the order of some microhenrys)connected in series with a spark-gap 27 (which might be replaced by adischarge tube of the ignitron or thyratron type) across the terminalsof a condenser 28 (of the order of 1 microfarad) capable of beingcharged, through a charge resistor 29, from a high voltage directcurrent source 30. Furthermore, the ends of coil 26 are connectedrespectively, one at 31 to metal tube 19 and the other at 32 to the wallof chamber 2, that is to say to a part at the reference potential R.

Finally, in order to facilitate extraction of the ions from bulb 22, wemay provide an extraction electrode 4 connected, through a conduit 33passing through the wall of chamber '2 by means of an insulating member4 to a terminal 34 of source 11 so that its potential ranges betweenthat of the wall of chamber 2 and that of the accelerating electrodes 5,6, 7, 8, being preferably closer to that of the wall of chamber 2.

The operation of the device above described to produce and to acceleratepositive ions (the polarities of sources 30 and 11 being as indicated byFIG. 1) is as follows.

Concerning first the ion source, there is produced through tubes 18 and19 and bulb 22 a small but continuous stream of the gas or vapor to beionized. The high voltage of source 30 charges condenser 28 throughresistor 29 until the voltage across the terminals of spark-gap 27 ishigher than the breakdown potential corresponding to the distancebetween the electrodes thereof. Condenser 28 then discharges into coil26 within a very short time in the form of a current pulse of highintensity, thus producing in bulb 22 an intensive electromagnetic fieldwhich ionizes and heats the gas flowing through bulb 22. This ionizedgas expands and therefore tends to escape very quickly through theoutlet 23, the throttled portion 20 of tube 19 preventing the gas fromflowing back toward flexible tube 18. A column of plasma (stronglyionized gas) is therefore formed between tube 19 and ring 24. As thisplasma is equivalent to an electric conductor branched across theterminals of the high voltage source between 24 and 31 (both ends ofcoil 26 being connected to tube 29 and 31 and to the wall of chamber 2at 32), this plasma is subjected, according to the law of Lenz, to anelectromagnetic force which facilitates the outflow of plasma throughnozzle 23 while imparting thereto an important acceleration in thedirection of arrow F.

The extraction electrode 4, which is at a negative potential withrespect to the ions and electrons of the plasma thus produced, furtherincreases the force with which the positive ions are ejected from bulb22, while rejecting the electrons toward the inside of the bulb.

As for the accelerator proper, which constitutes the subject matter ofthis invention, the positive ions which are substantially at thereference potential R are accelerated, after they have been extractedfrom bulb 22, toward electrode the potential of which is stronglynegative. When they get close to electrode 5, there is produced aphenomenon analogous to a variation of capacity, the bunch of positiveions and the negative electrode 5 at very high voltage constituting twoelements behaving like the plates of a capacitor. Considering thecircuit formed by the high voltage source 11, resistor 13 and thecapacitor-like structure thus defined, any variation of the value of thecapacity of this capacitor-like structure involves a variation of thecharacteristics of said circuit so that, when the ions come close toelectrode 5, the charges flow through resistor 13, which connectselectrode 5 to source 11. Consequently, the potential of electrode 5decreases in absolute value.

When the bunch of positive ions strikes electrode 5, it causes acomplete discharge of said electrode. Electrode 5 remains dischargeduntil the whole bunch of positive ions has passed therethrough.Therefore, at this time, there is produced between this electrode 5 andthe next electrode 6, a potential diflerence equal to the high voltageof source 11. This diflference of potential imparts to the bunch of ionsa new acceleration. Considering a series of successive acceleratingelectrodes such as 5, 6, 7, 8, it will be understood that the totalacceleration imparted to the positive ions is substantially equal to theproduct of the voltage applied to each electrode, as all electrodessuccessively traversed by one bunch of positive ions remain dischargeduntil the head of said bunch has reached electrode 8 by the number ofelectrodes.

Furthermore, the creation of a potential difference between theelectrode struck by the bunch of ions and the next electrode beingwholly independent of any external synchronizing means, since it isproduced just when the ions strike the electrode, the device accordingto the present invention is self-regulating.

Of course, when it is desired to produce and to accelerate negativeions, the polarities of the voltage sources 11 and 30 are reversed butthe operation remains unchanged.

In the case of linear accelerators of the type exemplified in thedrawings, it may be advantageous in some cases to dispose the successiveaccelerating electrodes 5, 6, 7, 8, at increasing distances from oneanother in the direction of diaphragm 9 so that the time of transit, ortime of flight, of the ions between two accelerating electrodes issubstantially constant, account being taken of the increase of velocityof the ions due to their successive accelerations by electrodes 5, 6, 7,8.

By way of example, we will indicate the following magnitudes for theelements of the device shown by FIG. 1:

(I) Pulsating Ion Source Flow rate of gas G: 1 cm. per minute atatmospheric pressure, the gas being hydrogen or deuterium to obtainprotons or deuterons respectively,

Volume of bulb 22: cm.

Source 30: 10,000 volts,

Condenser 28: 0.2 microfarad,

Coil 26: 10 microhenrys,

Frequency of the discharges of condenser 28: 50 discharges per second,

Duration of a discharge impulse: about 1 microsecond,

Energy of every impulse: about 10 joules,

Instantaneous power of a discharge: about 10 megawatts,

Ionic intensity at the output of source S: 10 amperes,

Difference of potential between 17 and 34: 20,000 volts.

(11) Linear Accelerator Proper Length of the accelerator: 1 meter,

Pressure in chamber 2: 10 mm. of mercury,

Number of accelerating electrodes: 10,

Nature of the accelerating electrodes: gride g constituted by nickelwires 1 occupying about 10% of the area of every grid,

Source 11: 100,000 volts,

Resistors 13 to 16: 1 megohm,

Peak current flowing through resistors 13 to 16: about 100 ma.,

Loss of ionic intensity for the whole of the ten acceleratingelectrodes: 1 ampere,

Acceleration obtained: it corresponds to an electrostatic potential ofabout 1 million volts in a conventional electrostatic accelerator of theVan de Graatf type.

We thus obtain an ion accelerator which, making use of a source ofrelatively moderate value (100,000 volts), permits of obtaining ions ofvery high energy (corresponding to an acceleration normally achieved bya difference of potential of about 1 million volts), this acceleratorhaving a self-regulating operation and being therefore capable ofaccelerating without any modification of structure ions of differentnature and charge, whether these ions are positive or negative.

In a general manner, while we have, in the above description, disclosedwhat we deem to be practical and efiicient embodiments of our invention,it should be Well understood that we do not wish to be limited theretoas there might be changes made in the arrangement, disposition and formof the parts without departing from the principle of the presentinvention as comprehended within the scope of the accompanying claims.

For instance, the invention might be applied in the case of anaccelerator of circular type including a series of electrodes such as 5,6, 7, 8, placed in an evacuated chamber in the form of a ring ofrectangular cross-section of relatively low axial height (as in the caseof cyclotrons) or of toroidal shape (as in the case of synchrotrons);means being provided to form a magnetic field parallel to the axis ofrevolution of the ring or of the tore so as to curve the path of travelof the ions respectively in spiral or circular shape. Of course, when itis desired to keep the ions on circular trajectories, it is necessarygradually to increase the intensity of the magnetic field as in the caseof synchrotrons.

In such circular accelerators, the ions would travel a great number oftimes through the evacuated chamber between the ion source and thetarget, every electrode 5, 6, 7, 8, acting several times as acceleratingelectrodes in the manner above described.

What we claim is:

1. An ion accelerator, for accelerating bunches of ions supplied at areference potential by a high intensity pulsating ion source, thisaccelerator comprising, in combination, an evacuated tubular chamberhaving one portion thereof connected with the outlet of said ion sourceso as to receive ions therefrom, a series of electrodes partly permeableto ions, said electrodes being disposed behind one another in saidchamber transversely to the axis thereof, a high direct voltage sourcehaving a first terminal at said reference potential and a secondterminal, a series of resistors of substantially identical highresistance and of negligible inductance having each one end connected toone of said electrodes, respectively, and conductor means for connectingsaid second terminal of said source to all the other ends of saidresistors.

2. An ion accelerator according to claim 1 in which the voltage of saidsource averages some tens of kilovolts and each of said resistors has aresistance of an order of magnitude of 1 megohm.

3. An ion accelerator according to claim 1 further comprising anextraction electrode located between said portion of the tubular chamberand that of said electrodes which is nearest thereto, and a conductorfor bringing said extraction electrode to a potential ranging betweenthose of said first and said second terminal of said high voltage sourceand closer to the potential of said first terminal than to the potentialof said second terminal.

4. An ion accelerator according to claim 3 in which said extractionelectrode is connected to an intermediate tap of said high voltagesource.

5. An ion accelerator according to claim 1 further characterized in thatsaid partly permeable electrodes are grids.

6. An ion accelerator according to claim 1 in which said partlypermeable electrodes are plates provided with a central hole.

7. An ion accelerator according to claim 1 in which said evacuatedtubular chamber is rectilinear, said portion of said chamber beinglocated at one of the ends thereof.

8. An ion accelerator according to claim 7 in which the distance betweentwo successive electrodes increases in the direction of the other end ofsaid tubular chamber.

9. An ion accelerator according to claim 1, in which said tubularchamber is made of an electricity conducting material and said chamberis connected to ground and to said first terminal.

10. An ion accelerator, for accelerating succesive copious bunches ofions supplied at a reference potential at the outlet of a pulsating ionsource, comprising in combination: an evacuated tubular rectilinearchamber constituted by a cylinder having a length of about one meter andbeing located around a geometrical axis, said chamber having one endthereof connected to the outlet of said source so as to receivetherefrom successive copious bunches of ions; a potential source ofabout hundred thousand volts having a first terminal maintained at saidreference potential and a second terminal; a series of substantiallyidentical electrodes partly permeable to said ions, the number of saidelectrodes being of the order of ten and said electrodes being disposedbehind one another in said chamber transversely to said geometrical axisof said cylinder; a series of resistors, the number of resistors beingequal to the number of electrodes and each of said resistors having aresistance of about one megohm and a negligible inductance, each of saidresistors including a first end connected to one of said electrodesrespectively and a second end; and conductor means of negligibleimpedance connecting said second end of each of said resistors to thesecond terminal of said potential source.

11. An ion accelerator according to claim 10, in which each of saidelectrodes is a grid.

12. An ion accelerator according to claim 10, in which each of saidelectrodes is a plate pierced by a central hole, the holes of said platebeing disposed along said geometrical axis of said cylinder.

13. An ion accelerator according to claim 10, in which said cylinder ismade out of an electricity-conducting material and is connected to saidfirst terminal of said potential source and to ground.

14. An ion accelerator according to claim 10, further comprising anextraction electrode located around said geometrical axis between saidend of said chamber and that one of said series of partly permeableelectrodes which is nearest thereto, and a conductor connecting saidextraction electrode to an intermediate tape of said potential sourcelocated between said first terminal maintained at said referencepotential and said second terminal thereof.

References Cited in the file of this patent UNITED STATES PATENTS2,261,569 Schutze Nov. 4, 1941 2,611,878 Coleman Sept. 23, 19522,688,088 Berry et al Aug. 31, 1954 2,770,755 Good Nov. 13, 19562,845,571 Kazan July 29, 1958 2,880,356 Charles et a1. Mar. 31, 1959

1. AN ION ACCELERATOR, FOR ACCELERATING BUNCHES OF IONS SUPPLIED AT AREFERENCE POTENTIAL BY A HIGH INTENSITY PULSATING ION SOURCE, THISACCELERATOR COMPRISING, IN COMBINATION, AN EVACUATED TUBULAR CHAMBERHAVING ONE PORTION THEREOF CONNECTED WITH THE OUTLET OF SAID ION SOURCESO AS TO RECEIVE IONS THEREFORM, A SERIES OF ELECTRODES PARTLY PERMEABLETO IONS, SAID ELECTRODES BEING DISPOSED BEHIND ONE ANOTHER IN SAIDCHAMBER TRANSVERSELY TO THE AXIS THEREOF, A HIGH DIRECT VOLTAGE SOURCEHAVING A FIRST TERMINAL AT SAID REFERENCE POTENTIAL AND A SECONDTERMINAL, A SERIES OF RESISTORS OF SUBSTANTIALLY IDENTICAL HIGHRESISTANCE AND OF NEGLIGIBLE INDUCTANCE HAVING EACH ONE END CONNECTED TOONE OF SAID ELECTRODES, RESPECTIVELY, AND CONDUCTOR MEANS FOR CONNECTINGSAID SECOND TERMINAL OF SAID SOURCE TO ALL THE OTHER ENDS OF SAIDRESISTORS.